Failure classification and cause analysis of lithium battery.

1、 Capacity decay failure
During the standard cycle life test, the discharge capacity shall not be less than 90% of the initial capacity when the number of cycles reaches 500. Or when the number of cycles reaches 1000, the discharge capacity shall not be less than 80% of the initial capacity. If the capacity drops sharply within the range of standard cycles, it belongs to capacity attenuation failure.

The failure of battery capacity attenuation is rooted in the failure of materials and is closely related to objective factors such as the battery manufacturing process and battery service environment. From the point of view of materials, the important causes of failure include the structural failure of cathode materials, the transition growth of SEI on the surface of the cathode, the decomposition and deterioration of electrolyte, fluid collector corrosion, trace impurities in the system, etc.

Structural failure of cathode materials: structural failure of cathode materials includes particle breakage, irreversible phase transformation, material disorder, etc. During the charging and discharging process, the structure of LiMn2O4 will be distorted due to the Jahn teller effect, and even particles will be broken, failing electrical contact between particles. Limn1.5ni0.5o4 material will undergo a tetragonal cubic phase transition during charging and discharging. During charging and discharging, co will enter the Li layer due to the transition and detachment of Li in LiCoO2 material, resulting in the disorder of layered structure and restricting its capacity.

Negative electrode material failure: the failure of graphite electrodes mainly occurs on the graphite surface. The graphite surface reacts with electrolyte to produce a solid electrolyte interface phase (SEI). If it grows excessively, the lithium-ion content in the internal system of the battery will be reduced, resulting in capacity attenuation. The failure of silicon anode materials is mainly due to the cycle performance problems caused by its huge volume expansion.

Electrolyte failure: LiPF6 has poor stability and is easy to decompose, reducing the content of transportable li+ in the electrolyte. It is also easy to react with the trace water in the electrolyte to form HF, causing corrosion inside the battery. Poor air tightness leads to deterioration of the electrolyte, changes in the viscosity and chromaticity of the electrolyte, and eventually lead to a sharp decline in the ion transport performance.

Failure of fluid collector: corrosion of fluid collector and decrease of fluid collector adhesion. HF generated by the failure of the above electrolyte will cause corrosion to the current collector and generate compounds with poor conductivity, increasing ohmic contact or the failure of active substances. During charging and discharging, the Cu foil is dissolved at a low potential and deposited on the surface of the positive electrode, which is called copper precipitation. The common form of collector failure is that the binding force between the collector and the active material is not enough, resulting in the stripping of the active material, which can not supply capacity for the battery.

2、 Internal short circuit

Internal short circuit often causes self-discharge, capacity attenuation, local thermal runaway, and safety accidents of lithium-ion batteries.

Short circuit between copper/aluminum current collectors: it is caused by the positive and negative current collectors' contact due to the penetration of the diaphragm or electrode by untrimmed metal foreign matters during the production or use of the battery, and the displacement of the pole piece or lug in the battery pack.

Short circuit caused by diaphragm failure: diaphragm aging, diaphragm collapse, diaphragm corrosion, etc. will lead to diaphragm failure. The failed diaphragm will lose electronic insulation or the gap will become positive and negative micro contact, and then there will be serious local heating. Continued charging and discharging will spread around, resulting in thermal loss of control.

Short circuit caused by impurities: if the transition metal impurities in the positive electrode slurry are not removed, the diaphragm will be punctured or the negative lithium dendrite will be generated, resulting in an internal short circuit.

Short circuit caused by lithium dendrites: lithium dendrites will appear where the local charge is uneven in the long cycle process, and the dendrites will cause an internal short circuit through the diaphragm.

In the process of battery design and manufacturing or battery pack assembly, unreasonable design or excessive local pressure will also lead to internal short circuits. Internal short circuits will also occur under the induction of battery overshoot and over-discharge.

3、 Increased internal resistance

The increase in the internal resistance of the lithium-ion battery will be accompanied by a decrease in energy density, voltage and power, battery heat generation, and other failure problems. The important factors leading to the increase of internal resistance of lithium-ion batteries are divided into key battery materials and battery service environments.

Key materials of battery: microcrack and breakage of positive material, damage of negative material and excessive surface SEI, aging of electrolyte, separation of active material from the collector, poor contact between the active material and conductive additives (including loss of conductive additives), blockage of diaphragm shrinkage cavity, abnormal welding of battery lug, etc.

Battery use environment: high/low ambient temperature, overcharge and discharge, high rate charge and discharge, manufacturing process and battery design structure, etc.

4、 Gas production

During the battery formation process, the gas production phenomenon caused by consuming electrolytes to form a stable SEI film is normal gas production, but the excessive consumption of electrolytes to release gas or the release of oxygen from cathode material is abnormal gas discharge. It often occurs in soft-packed batteries, which may cause excessive internal pressure and deformation of the battery, rupture of the encapsulated aluminum film, internal cell contact problems, etc.